1. Field of the Invention
The invention relates to a printhead structure in a direct electrostatic printing device in which printing is carried out by selectively conveying charged toner particles from a particle source directly onto an image receiving substrate. More specifically, the invention relates to a printhead structure including means for focusing the streams of toner particles conveyed from the particle source towards the image receiving substrate, thereby considerably reducing the dot size of the printed image, resulting in increased print resolution and improved print quality.
2. Description of the Related Art
Of the various electrostatic printing techniques, the most familiar and widely utilized is xerography, wherein latent electrostatic images formed on a charge retentive surface, such as a roller, are developed by a toner material to render the images visible, the images being subsequently transferred to plain paper. This process is called an indirect process since the visible image is first formed on an intermediate photoreceptor and then transferred to a paper surface.
Another method of electrostatic printing is one that has come to be known as direct electrostatic printing, DEP. This method differs from the aforementioned xerographic method in that charged toner particles are deposited directly onto an information carrier to form a visible image. In general, this method includes the use of electrostatic fields controlled by addressable electrodes for allowing passage of toner particles through selected apertures in a printhead structure. A separate electrostatic field is provided to attract the toner particles to an image receiving substrate in image configuration.
The novel feature of direct electrostatic printing is its simplicity of simultaneous field imaging and toner transport to produce a visible image on the substrate directly from computer generated signals, without the need for those signals to be intermediately converted to another form of energy such as light energy, as is required in electrophotographic printers, e.g., laser printers.
U.S. Pat. No. 5,036,341 granted to Larson discloses a direct electrostatic printing device and a method to produce text and pictures with toner particles on an image receiving substrate directly from computer generated signals. According to that method, a control electrode array is positioned between a back electrode and a rotating particle carrier. An image receiving substrate, such as paper, is then positioned between the back electrode and the control electrode array.
An electrostatic field from an electric potential on the back electrode attracts the toner particles from the surface of the particle carrier to create particle streams toward the back electrode. The particle streams are modulated by voltage sources which apply an electric potential to selected control electrodes of the control electrode array to create electric fields which permit or restrict transport of toner particles from the particle carrier. In effect, these electric fields open or close selected apertures in the control electrode array to the passage of toner particles by influencing the attractive force from the back electrode. The modulated streams of charged particles allowed to pass through selected apertures impinge upon an image receiving substrate interposed in the particle stream to provide line-by-line scan printing to form a visible image.
The control electrode array described in the above-mentioned patent is in the form of a lattice of individual wires arrange in rows and columns. A control electrode array operating according to the described principle may, however, take on any one of several other designs. Generally, the array is a thin sheet-like element, referred to as a Flexible Printed Circuit or FPC, comprising a plurality of addressable control electrodes and corresponding voltage signal sources connected thereto for attracting charged toner particles from the surface of a particle carrier to an image receiving substrate. A sequence of electronic signals, defining the image information, is converted into a pattern of electrostatic fields which locally modify the uniform field from the back electrode, thereby selectively permitting or restricting the transport of charged particles from the particle carrier and producing an image pattern corresponding to the electrostatic field pattern onto the image receiving substrate.
A flexible control array or FPC as discussed in, for example, U.S. Pat. No. 5,121,144, also granted to Larson, is made of a flexible, electrically insulated, nonrigid material, such as polyimide, or the like, which is provided with a multiplicity of apertures and overlaid with a printed circuit whereby the apertures in the material are arranged in rows and columns and surrounded by ring-shaped electrodes. A uniform electrostatic field generated by a back electrode attracts toner particles from a particle carrier to create a particle stream through the FPC toward the back electrode. All control electrodes are initially at a white potential V.sub.w which means that particle transport from the particle carrier toward the back electrode is inhibited. As image locations on an image receiving substrate pass beneath the apertures, selected control electrodes are set to a black potential V.sub.b to produce an electrostatic field drawing the toner particles from the particle carrier. Charged toner particles allowed to pass through the apertures in the FPC are subsequently deposited on the substrate in the configuration of the desired image pattern. The toner particle image is then made permanent by using heat and pressure to fuse the toner particles on the surface of the substrate.
Print resolution is determined by the number of distinguishable dots that can be printed per length unit across the image receiving substrate. Therefore, to meet the requirements of increased print resolution, it is essential to improve the control function in order to provide sufficiently small dots. For instance, to obtain a print resolution of 600 dots per inch (DPI), the overlap width of two adjacent dots might not exceed 1/600 inch, i.e., about 42 microns, and the size of a distinct dot might be in the order of 60 to 80 microns.
A drawback of the above-mentioned method is that the electrostatic fields controlling toner transport are not sufficiently convergent to create as small dots as are required for higher resolution print. In effect, the aperture size is typically in the order of 100 to 150 microns in diameter. The dot size can be decreased by reducing the amount of toner passing through the aperture to provide dots which are significantly smaller than the aperture. However, that may not only influence the dot size, but even considerably alter the dot density and uniformity.
Another drawback of the above-mentioned method is that, although the control electrodes have preferably a symmetric shape about a central axis of the apertures, the electric field configuration in the vicinity of an aperture may not be perfectly symmetrical due to interaction with adjacent connectors joining the control electrodes to the control unit, resulting in that toner particles may be slightly deflected from their initial trajectory, forming displaced dots on the image receiving substrate.
Another drawback of the above-mentioned method is that an electrostatic field generated by a control electrode may influence other apertures than the intended aperture, causing undesired dot size variation due to the neighboring field configuration (cross-talk).
Therefore, Applicant has perceived a need to improve a printhead structure in order to obtain dot sizes that satisfy the requirements of higher print resolution, for instance, a print resolution of 600 DPI or higher, and eliminates or at least considerably reduces dot deflection and cross-talk.